A transfer system used in a production factory, such as a production factory of semiconductor devices, has been known. For example, the transfer system includes processing equipments for processing objects, such as semiconductor wafers. The transfer system controls transfer of the objects (hereinafter referred to as the processing objects) to the processing equipments so that processing steps are sequentially performed to the processing objects in the processing equipments.
In such a transfer system, the processing objects are generally accommodated in cassettes to prevent the processing objects from being contaminated. The cassettes are carried to and collected from the processing equipments by carriers. Each of the carriers is assigned with a predetermined transfer command, The carrier transfers the cassette from one location to another location in accordance with the predetermined transfer command.
Such a transfer system further includes a storage to temporarily store the cassette. The cassette is stored once in the storage when a subsequent processing equipment to which the cassette is to be transferred next to have a subsequent processing step is processing the processing object of another cassette. When the subsequent processing equipment finishes the processing, the cassette processed is collected from the subsequent processing equipment, and the cassette that has been stored in the storage is transferred to the subsequent processing equipment to have the subsequent processing step.
In recent years, such a transfer system has been required to shorten a lead time. For example, JP 2002-110766 A describes to assign a transfer command to the carrier irrespective to whether the carrier is available to transfer the cassette. As another example, JP 2009-135275 A describes to assign a transfer command to the carrier such that the carrier arrives at the processing equipment on the time the processing of the processing equipment finishes. In the transfer systems described, however, the following issues are concerned. For example, in a general production factory of the semiconductor devices, only several or several tens of carriers are used whereas there are several hundreds of processing equipments, Therefore, it is actually difficult to transfer the carriers relative to all the processing equipments in the above described manner.
Also, periodic inspection and repairs are generally performed for the processing equipments. For example, when the processing equipment has periodic inspection or a repair, a large number of the cassettes accommodating the processing objects to be processed by this processing equipment are likely to be accumulated in the storage. In the transfer system described, the carrier is transferred to the processing equipment on time the processing finishes in the similar manner to all the processing equipments. Therefore, there is a possibility that the carrier will not be carried to the processing equipment after the periodic inspection or the repair. In such a case, the number of the cassettes accumulated in the storage increases, and cassettes, which have not been processed by the processing equipment having the periodic inspection or the repair, cannot be transferred to the storage. That is, in the transfer system described, when the processing equipment has the periodic inspection or the repair, transferring balance is likely to be deteriorated, resulting in a decrease in an operating ratio and an increase in the lead time.